1. Field of the Invention
The present invention relates to a surface acoustic wave device having plural sets of parallel-connected input and output transducers arranged on a piezoelectric substrate such that a loss is minimized.
2. Description of the Prior Art
A conventional device of this type, as shown in FIG. 1 has parallel-connected input interdigital transducers 2a and 2b and parallel-connected output interdigital transducers 3a and 3b arranged on a piezoelectric substrate 1 with the input transducers 2a and 2b being connected in parallel by common bus bars 4 and 5 and to an input signal line 6 and the output electrodes 3a and 3b being connected in parallel by common bus bars 7 and 8 and to an output signal line 9 from which an output signal is taken. Numeral 10 denotes an acoustic wave absorber. The plurality of input and output transducers are arranged to allow the coverage of a wide band by selecting different operating frequency bands of the respective transducers. The common connections by the bus bars allow the size reduction of the entire device.
In the surface acoustic wave device having the parallel-connected input and output transducers as shown in FIG. 1, the plurality (two in the illustrated example) of input and output transducers are respectively designed to present optimum frequency characteristics. Accordingly, when an electric signal is applied to the input signal line 6, a characteristic in a certain frequency band is affected by impedances of other transducers which contribute to other frequency bands so that a loss tends to increase. For example, in a transducer structure shown below, the loss of the transducer set 2a and 3a increases by approximately 1.5 dB by the parallel connection while the loss of the transducer set 2b and 3b increases by approximately 7.0 dB by the parallel connection.
______________________________________ Center Aperture Static Trans- Number of Frequency dimension capacitance ducer Electrodes (MHz) (Microns) (pF) ______________________________________ 2a 123 Weighted W.sub.1 = 600 2.8 3a 14 602 W.sub.2 = 600 1.7 2b 10 752 W.sub.3 = 600 1.2 3b 10 752 W.sub.4 = 600 1.2 ______________________________________